An inertia measurement sensor or a control sensor which is used in a vehicle traveling control device or a robot posture control device is manufactured by, for example, a micro-electro-mechanical systems (MEMS) technique. A sensor which measures an inertia using a variation in capacitance has been known as an example of the inertia measurement sensor.
Among inertial sensors, there is a so-called combined sensor which can detect acceleration and an angular velocity, which are kinds of the inertia, at the same time. For example, the following technique is used to achieve the combined sensor.
With the progress of a semiconductor process technique and a micro-machining technique (so-called MEMS technique), a MEMS combined sensor element which includes a detection circuit and detects an inertia using the detection circuit has come into widespread use. For example, an antiskid brake system of the vehicle detects the rotation of the vehicle and acceleration applied to the vehicle in all directions using an angular velocity sensor and a biaxial acceleration sensor and adjusts an engine output and the braking force of four wheels to control the traveling state of a moving body. In general, as the sensors used in the antiskid brake system of the vehicle, one angular velocity sensor and acceleration sensors with a plurality of detection axes are mounted on a printed circuit board. In some cases, in order to meet a demand for low manufacturing costs, a plurality of sensor elements are incorporated into one sensor element to reduce manufacturing costs or mounting costs.
An example of the technique related to the inertial sensor is disclosed in the following PTL 1.
When a silicon wafer is used to form the MEMS combined sensor, it is processed by a silicon deep etching technique. The deep etching technique is a processing technique which repeatedly performs chemical etching using SF6 gas as a main component and chemical deposition using CF4 gas as a main component. A needle-shaped silicon residue which is called black silicon is generated in a portion with a large etching area by the influence of the uniformity of deposition or the local charge-up of ions (a variation in potential due to the flow of charge) and becomes conductive dust. The dust is not preferable in terms of the characteristics of the sensor.
In addition, when a region in which the aspect ratio of a processing portion which is defined by the ratio of the thickness and the gap of the silicon wafer is greatly different is generated in the plane of the wafer due to a difference in the entrance state of gas or ions, a variation in the etching rate occurs. This phenomenon is called a micro-loading effect, in which the etching rate is reduced as an opening portion of an etched region is reduced. The time required to complete deep etching for a fine pattern portion is increased by the micro-loading effect.
A variation in the time required to complete etching causes a processing pattern portion other than the fine pattern requiring the longest time to be exposed to a chemical, which is an etchant, even though the etching of the processing pattern portion has been completed, and the processing pattern portion is over-etched. As a result, there is a difference between the dimensions of the top of the inertial body and the dimensions of the bottom of the inertial body and the inertial body cannot be processed as designed, which is not preferable. In order to prevent a variation in the opening of the etched region to suppress a variation in the processed dimensions, a dummy pattern which does not directly contribute to the sensor performance is provided around an inertial mass body, a movable electrode and a fixed electrode forming a detection electrode, and a support beam structure pattern in the same plane.
Conductive portions, such as the dummy pattern which is provided in order to reduce conductive dust or control the variation in the processed dimensions, a supporting layer for fixing and supporting the inertial mass body, the detection electrode, the support beam, and the dummy pattern, and a lid for surrounding or covering these components, are referred to as ‘peripheral conductors’. The peripheral conductors include single-crystal silicon or an insulating film and a conductive film which are formed on the single-crystal silicon. In addition, when the inertial mass body, the detection electrode, the support beam, and the peripheral conductors are made of single-crystal silicon, a natural oxide film with a thickness of about several nanometers is formed on the surface of the processed peripheral conductors.
The peripheral conductors including the dummy pattern provided around the inertial mass body or the support beam need to be fixed to a predetermined potential in order to prevent charge-up due to electromagnetic waves or static electricity. However, when the peripheral conductors are fixed to a potential different from the potential of movable portions, such as adjacent inertial mass bodies, electrostatic attractive force is generated between the peripheral conductors and the movable portions. As a result, for example, defects, such as an offset, a variation in sensitivity, sticking, and a short circuit, occur in the sensor. Therefore, it is preferable that the peripheral conductors and the movable portions adjacent to the peripheral conductors have the same potential.
The following PTL 2 discloses an example of the technique related to the inertial sensor.